Retrieve content's digest.
The digest will be compared with locally stored value to see if
content has changed and must be downloaded again.
Implementation is expected not to download whole content to get the digest.
url - source url of the artifact to retrieve digest for
the digest or null, if the digest can not be retrieved by this transport
Interrupts current transfer operation as early as possible. Will be called from external thread,
so implementation should be thread-safe.
Main thread is expected to throw ResolvingInterruptedException
when interrupt is received